As was explained for protons on oxygen, protons on nitrogen exchange quickly enough to be uncoupled from other protons in the molecule.
To observe coupling between amino protons and other protons in the molecule, the pH of the experiment can be reduced (pH < 1). This will slow down the exchange rate of the amino protons, and coupling with other protons in the molecule can then be observed.
Due to the partial double bond character of C-N bonds in amides, groups attached to amide nitrogens are in different magnetic environments. Therefore, even identical groups on nitrogen will give different signals. Increasing the temperature of the experiment will allow free rotation about the C-N bond, and the groups will become magnetically equivalent (and give only one signal).
Protons directly attached to nitrogen in amides are frequently decoupled from all other protons in the structure (see above), and give singlet peaks. These peaks usually appear to be broad, and this is due to quadropole broadening from the nitrogen nucleus.
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